dropwatch: fix static link

dropwatch: fix static link

Use LDLIBS for the list of libraries, since LDFLAGS appear before the source
files in the link command, and the upstream LDLIBS lacks -lncurses which is a
dependency of readline. We already pass target LDFLAGS in
TARGET_CONFIGURE_OPTS, so drop that part.

Fixes:
http://autobuild.buildroot.net/results/ce4/ce436bdd7e7950c801edac01f311b89110d87d62/
http://autobuild.buildroot.net/results/691/691d61d13849150c3616b1c74c964f50118ff1d9/
http://autobuild.buildroot.net/results/291/2918c75e473c5a2434e72f2e409becc5b4d07908/

Cc: Tzu-Jung Lee <roylee17@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>